The usual shut-off devices of the type referred to include plungers or pins which may be displaceable either transversely to the flow path or longitudinally thereof; see, for example, U.S. Pat. Nos. 4,073,944 and 4,076,485. Conventional mechanisms used for this purpose have the drawback that the channel carrying the plastic flow to the nozzle orifice has discontinuities forming dead-air pockets in which the plastic mass tends to accumulate and degrade while being bypassed by the mainstream of that mass. Some of these mechanisms, moreover, are rather complicated and require an enlarged nozzle body calling for extra heating energy to keep the mass fluid.